This invention relates to a printed circuit board on which an electronic component such as a semiconductor element is mounted. In particular, the present invention relates to a multilayer printed circuit board.
In a recent electronic device having a multilayer printed circuit board and an electronic component mounted thereon, a high-frequency current flows into a power-supply layer or a ground layer of the multilayer printed circuit board because of various reasons. For example, electromagnetic coupling between a power-supply layer and a signal layer causes a high-frequency current to flow into the power-supply layer. A weakened ground layer causes a high-frequency current to flow into the ground layer. A length of a return-current path also influences a high-frequency current flowing into the ground layer. The flow of the high-frequency current into the power-supply layer or the ground layer constitutes a large current-loop which may cause a noise radiation problem.
Since such noise is radiated from an inner layer of a multilayer printed circuit board, a noise suppression sheet stuck on an outer surface of the multilayer printed circuit board has little effect on reduction of the noise.
JP-A 2006-100608 or JP-A 2006-019590 discloses a pre-impregnation sheet that is, at least in part, formed of magnetic material, those documents being incorporated herein by reference. A multilayer printed circuit board formed of the pre-impregnation material can suppress the above-mentioned noise radiated from the inside thereof. However, in order to obtain desirable noise suppression property in practical use, the pre-impregnation material with the magnetic material becomes very thicker. The thicker pre-impregnation material results in a larger multilayer printed circuit board larger. Therefore, there is a need for a new structure that can suppress the above-mentioned noise with small size.